The invention relates to the field of surgical implants intended to impede the coming-together of two vertebrae during movements of the spinal column.
When intervertebral discs have been subject to considerable wear or degeneration, they may become powerless to prevent excessive coming-together of two successive vertebrae during flexion (forward movement) or extension (backward movement) of the spinal column. Such coming-together may have the consequence of crushing the spinal nerves, which is very painful for the patient. In the most serious cases, this coming-together may continue until the vertebrae contact one another during extreme movements.
A first known method for solving this problem consists in fastening one of the vertebrae in question to the other, so as to keep a space between them which is permanently enough. This fastening may be carried out by means of various devices in the form of plates. However, it causes a significant loss of mobility for the patient, because it prohibits any flexion or extension of the spinal column in the region in question.
It is also known to ligature the spinous processes of the vertebrae in question by means of artificial ligaments. This makes it possible to retain freedom of movement in extension for the vertebrae while limiting the magnitude of the allowed flexions, to the extent of prohibiting contact between the front of the vertebrae. However, this does not prevent contact at the back of the vertebrae, although this region is where compression of the spinal nerves is most likely to occur.
It is also known to insert interspinous wedges between the spinous processes, as in document EP-A-0 392 124. These wedges are made of a rigid material such as PTFE, and thus prevent any contact between the spinous processes that they separate during extension of the spinal column. Preferably, they are held in place by artificial ligaments which pass through them and are wound around the spinous processes. By virtue of these ligaments, it is also possible to limit the magnitude of flexion movements of the spinal column in the region in question.
However, the fact of having a rigid element between the spinous processes leads to virtual removal of the extension movements of the spinal column at that point, or at least to a sudden interruption of such movements when the processes come into contact with the wedge.
It would be desirable to be able to use a device reproducing the normal behaviour of a vertebra-disc-vertebra stack (functional unit) more faithfully, where normally the action of blocking the movements of the spinal column only takes place gradually.
The aim of the invention is to provide a device meeting this requirement.